DESCRIPTION

These system calls change the owner and group of a file.
The
chown(),
fchown(),
and
lchown()
system calls differ only in how the file is specified:

*

chown()
changes the ownership of the file specified by
pathname,
which is dereferenced if it is a symbolic link.

*

fchown()
changes the ownership of the file referred to by the open file descriptor
fd.

*

lchown()
is like
chown(),
but does not dereference symbolic links.

Only a privileged process (Linux: one with the
CAP_CHOWNcapability) may change the owner of a file.
The owner of a file may change the group of the file
to any group of which that owner is a member.
A privileged process (Linux: with
CAP_CHOWN)
may change the group arbitrarily.

If the
owneror
groupis specified as -1, then that ID is not changed.

When the owner or group of an executable file is
changed by an unprivileged user, the
S_ISUIDand
S_ISGIDmode bits are cleared.
POSIX does not specify whether
this also should happen when root does the
chown();
the Linux behavior depends on the kernel version.
In case of a non-group-executable file (i.e., one for which the
S_IXGRPbit is not set) the
S_ISGIDbit indicates mandatory locking, and is not cleared by a
chown().

fchownat()

The
fchownat()
system call operates in exactly the same way as
chown(),
except for the differences described here.

If the pathname given in
pathnameis relative, then it is interpreted relative to the directory
referred to by the file descriptor
dirfd(rather than relative to the current working directory of
the calling process, as is done by
chown()
for a relative pathname).

If
pathnameis relative and
dirfdis the special value
AT_FDCWD,
then
pathnameis interpreted relative to the current working
directory of the calling process (like
chown()).

If
pathnameis absolute, then
dirfdis ignored.

The
flagsargument is a bit mask created by ORing together
0 or more of the following values;

AT_EMPTY_PATH (since Linux 2.6.39)

If
pathnameis an empty string, operate on the file referred to by
dirfd(which may have been obtained using the
open(2)
O_PATHflag).
In this case,
dirfdcan refer to any type of file, not just a directory.
If
dirfdis
AT_FDCWD,
the call operates on the current working directory.
This flag is Linux-specific; define
_GNU_SOURCEto obtain its definition.

AT_SYMLINK_NOFOLLOW

If
pathnameis a symbolic link, do not dereference it:
instead operate on the link itself, like
lchown().
(By default,
fchownat()
dereferences symbolic links, like
chown().)

VERSIONS

CONFORMING TO

The 4.4BSD version can be
used only by the superuser (that is, ordinary users cannot give away files).

fchownat():
POSIX.1-2008.

NOTES

Ownership of new files

When a new file is created (by, for example,
open(2)
or
mkdir(2)),
its owner is made the same as the filesystem user ID of the
creating process.
The group of the file depends on a range of factors,
including the type of filesystem,
the options used to mount the filesystem,
and whether or not the set-group-ID mode bit is enabled
on the parent directory.
If the filesystem supports the
-o grpid(or, synonymously
-o bsdgroups)
and
-o nogrpid(or, synonymously
-o sysvgroups)
mount(8)
options, then the rules are as follows:

*

If the filesystem is mounted with
-o grpid,
then the group of a new file is made
the same as that of the parent directory.

*

If the filesystem is mounted with
-o nogrpidand the set-group-ID bit is disabled on the parent directory,
then the group of a new file is made the same as the
process's filesystem GID.

*

If the filesystem is mounted with
-o nogrpidand the set-group-ID bit is enabled on the parent directory,
then the group of a new file is made
the same as that of the parent directory.

As at Linux 2.6.25,
the
-o grpidand
-o nogrpidmount options are supported by ext2, ext3, ext4, and XFS.
Filesystems that don't support these mount options follow the
-o nogrpidrules.

Glibc notes

On older kernels where
fchownat()
is unavailable, the glibc wrapper function falls back to the use of
chown()
and
lchown().
When
pathnameis a relative pathname,
glibc constructs a pathname based on the symbolic link in
/proc/self/fdthat corresponds to the
dirfdargument.

NFS

The
chown()
semantics are deliberately violated on NFS filesystems
which have UID mapping enabled.
Additionally, the semantics of all system
calls which access the file contents are violated, because
chown()
may cause immediate access revocation on already open files.
Client side
caching may lead to a delay between the time where ownership have
been changed to allow access for a user and the time where the file can
actually be accessed by the user on other clients.

Historical details

The original Linux
chown(),
fchown(),
and
lchown()
system calls supported only 16-bit user and group IDs.
Subsequently, Linux 2.4 added
chown32(),
fchown32(),
and
lchown32(),
supporting 32-bit IDs.
The glibc
chown(),
fchown(),
and
lchown()
wrapper functions transparently deal with the variations across kernel versions.

In versions of Linux prior to 2.1.81 (and distinct from 2.1.46),
chown()
did not follow symbolic links.
Since Linux 2.1.81,
chown()
does follow symbolic links, and there is a new system call
lchown()
that does not follow symbolic links.
Since Linux 2.1.86, this new call (that has the same semantics
as the old
chown())
has got the same syscall number, and
chown()
got the newly introduced number.

EXAMPLE

The following program changes the ownership of the file named in
its second command-line argument to the value specified in its
first command-line argument.
The new owner can be specified either as a numeric user ID,
or as a username (which is converted to a user ID by using
getpwnam(3)
to perform a lookup in the system password file).